US Patent Application Publication No. 2012/0271082 to Taylor et al., for “Variations on Prins-Like Chemistry to Produce 2,5-Dimethylhexadiene from Isobutanol” (hereafter, “Taylor et. al”), describes a method to produce 2,5-dimethylhexadiene from renewable isobutanol, from which in turn a renewable p-xylene (and subsequently, a renewable terephthalic acid, a key monomer in the production of PET) can be prepared. In addition, methods and catalysts are provided for producing 2,5-dimethylhexadiene from a variety of feed stocks that can act as “equivalents” of isobutylene and/or isobutyraldehyde including isobutanol, isobutylene oxide, and isobutyl ethers and acetals.
As background for the need of a source of renewable p-xylene, as summarized in Taylor et al., conventionally aromatic compounds such as para-xylene are produced from petroleum feedstocks in refineries by reacting mixtures of light hydrocarbons (C1-C6) and naphthas over various catalysts at high heat and pressure. The mixture of light hydrocarbons available to a refinery is diverse, and correspondingly provides a diverse mixture of aromatic compounds (e.g., BTEX benzene, toluene, ethylbenzene and xylenes, as well, as aromatic compounds having a molecular weight higher than xylenes). The xylenes product consists of three different aromatic C8 isomers: p-xylene, o-xylene, and m-xylene; typically about one third of the xylenes are the p-xylene isomer. The BTEX mixture is then subjected to subsequent processes to obtain the desired product. For example, toluene can be removed and disproportionated to form benzene and xylene, or the individual xylene isomers can be isolated by fractionation (e.g. by absorptive separation, fractional crystallization, etc.). Para-xylene is the most commercially important xylene isomer, and is used almost exclusively in the production of polyester fibers, resins, and films.
Alternatively, a single component feedstock purified from crude oil or synthetically prepared at the refinery can be selectively converted to a purer aromatic product. For example, pure isooctene can be selectively aromatized to form primarily p-xylene over some catalysts (see, for example, U.S. Pat. No. 3,202,725, U.S. Pat. No. 4,229,320, U.S. Pat. No. 4,247,726, U.S. Pat. No. 6,600,081, and U.S. Pat. No. 7,067,708), and n-octane purified from crude oil can be converted to primarily o-xylene (see for example, U.S. Pat. No. 2,785,209).
Very high p-xylene purity is required to prepare terephthalic acid of suitable purity for use in polyester production; typically at least about 95% purity, or in some cases 99.7% or higher purity of p-xylene is required. Conventional, processes for producing high purity p-xylene are thus complex and expensive: the conventional BTEX process requires isolation and extensive purification of p-xylene produced at relatively low levels; and alternative processes require isolation, and purification of single component feedstocks for aromatization from complex hydrocarbon mixtures. Furthermore, production of p-xylene from conventional petroleum-based feedstocks contributes to environmental degradation (e.g., global warming, air and water pollution, etc.), and fosters over-dependence on unreliable petroleum supplies from politically unstable parts of the world.
Taylor et al. acknowledges that it had previously been known that p-xylene can also be made from 2,5-dimethyl-2,4-hexadiene in a high yielding, clean reaction over chromia and other metal oxide catalysts (citing U.S. patent application Ser. No. 12/986,918 filed Jan. 7, 2011, now published as U.S. Pat. No. 8,450,543). It had also been earlier established that the 2,5-dimethyl-2,4-hexadiene could in turn be prepared by combining isobutene (or t-butanol, or a combination of isobutene and t-butanol) with isobutyraldehyde over a niobic oxide catalyst under acidic conditions, for example as taught in U.S. Pat. No. 4,684,758. The isobutene could be renewably sourced, obtained either from dehydrating isobutanol or from ethanol from biomass fermentation methods, an example method for producing isobutene from ethanol being described in Sun et al., “Direct Conversion of Bio-ethanol to Isobutene on Nanosized ZnxZryOz Mixed Oxides with Balanced Acid-Base Sites”, Journal, of the American Chemical Society, vol. 133, pp. 11096-11099 (2011). Unfortunately, however, the process of the '758 patent was said by Taylor et al. to require a discrete isobutyraldehyde feed prepared from other, non-renewable source starting materials such as propylene and formaldehyde, so that the “overall process entails multiple processing steps from multiple, different raw materials.”
Taylor et al. thus principally concerns methods for producing both of isobutylene (or isobutylene equivalents, e.g., isobutanol) and isobutyraldehyde (or isobutyraldehyde equivalents, e.g., isobutylene oxide) from isobutanol, and especially from renewable isobutanol, either in separate reactions or in situ with an olefin-aldehyde condensation reaction to form 2,5-dimethylhexadiene and/or 2-methyl-2,4-heptadiene. These synthons can then be coupled to form the desired dienes, which can then be cyclized to form o- and p-xylene. In certain favored embodiments, renewable isobutanol is the sole input chemical and is reacted under appropriate conditions to provide a product stream containing a desired fraction of isobutyraldehyde, e.g., via careful selection of oxidation catalysts and process conditions, and the partially oxidized product stream can then be reacted directly, eliminating the need for multiple feedstocks from both renewable and non-renewable sources.
At least one remaining difficulty with Taylor et al's approach, however, is that while a biosynthetic pathway to produce isobutanol has been known for some time using bacteria from the genus Clostridium, and while this pathway has been genetically engineered into a number of species of microorganisms more easily manipulated than those of the genus Clostridium, nevertheless, these engineered microorganisms had not achieved the ability to produce isobutanol in quantities large enough for commercial use. Peralta-Yahya, Pamela P.; Zhang, Fuzhong; del Cardayre, Stephen B.; Keasling, Jay D., “Microbial engineering for the production of advanced biofuels”, Nature, vol. 488 (7411): 320-328 (15 Aug. 2012).
A somewhat earlier published application to the same assignee and naming some of the same inventors, US 2011/0087000 to Peters et al. (hereafter, “Peters et al.”), is similarly directed to the production of renewable and relatively high purify p-xylene from biomass. The biomass is again treated to provide a fermentation feedstock and the fermentation feedstock then fermented with a microorganism for producing a C4 alcohol such as the aforementioned isobutanol. The isobutanol is then sequentially dehydrated in the presence of a dehydration catalyst to provide a C4 alkene such as isobutene. The C4 alkene is dimerized to form one or more C8 alkenes such as 2,4,4-trimethylpentenes or 2,5-dimethylhexene, then these materials are dehydrocyclized in the presence of a suitable dehydrocyclization catalyst to selectively form renewable p-xylene in high overall yield. The renewable p-xylene can then be oxidized to form terephthalic acid or terephthalate esters. Consequently, while the process steps in Peters et al. differ substantially from those in Taylor et al., nevertheless both are reliant on the same base technology for producing isobutanol by engineered microorganisms through fermentation.
It would, accordingly be advantageous if a process were available for making a renewable para-xylene that ultimately draws from a renewable feedstock that is already ubiquitous and inexpensive, and it would be particularly advantageous if this renewable feedstock also lent itself to the existing, known pathways to para-xylene from fossil fuel-based materials. We have now developed such a process.